1. Field of the Invention
The present invention relates to acoustic wave reflectors that work in a liquid environment, generally at sea, and may be subjected to high pressure corresponding to deep immersion.
2. Description of the Prior Art
Most acoustic wave reflectors work by using the reflection that takes place on an interface between the sea environment and another environment in which the speed of the acoustic waves is greatly different from their speed in water.
The ideal method would be to use a very thin plate that is perfectly transparent to acoustic waves and demarcates a volume in which a vacuum has been set up. In practice, metal strips of a certain thickness are used, letting through acoustic waves up to frequencies of the order of some kHz, and a filler gas which is most commonly just air. Naturally, such a device is highly sensitive to pressure, and there can be no question of submerging it to any significant depth.
The French patent No. 2 539 541, filed by the present applicant on Jan. 19, 1983, has proposed the use, instead of air, of a foam formed by a visco-elastic matrix enclosing a large number of gas bubbles. Improved performance values are thus obtained, but these are far from satisfactory.
For, the foam gets quickly crushed owing to the pressure, and the speed of sound increases accordingly therein, so to approach its speed in water. This considerably reduces the reflective power of the device. In practice, it then becomes difficult to exceed a submersion depth of the order of 100 m.
Furthermore, the visco-elastic material is fairly sensitive to temperature, and this also contributes to lowering the performance values of the device.